1. Field of the Invention
The present invention relates a radio communication system and, more particularly, to a discontinuous transmission (DTX) bit processing method for an adaptive multirate modulation.
2. Description of the Related Art
With the emergence of the information society, the center of services of telecommunication field is shifted from the low speed voice communication service to a multimedia service providing audio, video as well as voice communication. In order to support multimedia services that require high speed data transmission, it is critical to maximize the channel capacity by effectively utilizing the limited resources such as power and frequency band.
For effective utilization of the limited resources, various modulation techniques have been developed. In case of a wired network, channel characteristics are stable so that high efficiency QAM modulation/demodulation techniques are widely used. A radio network, generally, is not stable compared to the wired network, due to multipath transmission and fading according to a Doppler effect. A frequency shift keying (FSK) or a phase shift keying (PSK) modulation in which 1˜2 bits are transferred per 1 Hz. However, recently, with the development of the modulation technique, a multirate modulation scheme such as 16-QAM, 64-QAM, or the like is adopted even in the radio network.
The current and undergoing radio communication systems in the domestic and foreign countries are likely to adopt a fixed modulation scheme regardless of a channel state between the base station and the subscriber. Such a system has a problem in that that a low rate modulation scheme is used, even when a high rate modulation can be used in a good channel state, resulting in failure of maximization of the system capacity.
Also, the system adopting the fixed modulation scheme can not change the modulation rate such that the same low rate modulation is used even when the channel state is bad. Accordingly, the transmission quality is further degraded, and causes the communication channel to break.
In order to solve the above problems, the 3rd Generation Partnership Project (3GPP) standard conference has discussed a High speed Downlink Packet Access (HSDPA) on the basis of an adaptive modulation.
In the quadrature phase shift keying (QPSK) modulation, the DTX bit can be allocated to one of In-Phase (I) and quadrature (Q) axes in the IQ plane so that the transmission rate can be decreased by turning off the transmission power to the axis to which the DTX bit is allocated.
Since the discussion on the HSDPA proceeded, the multirate modulation scheme such as the QAM (Quadrature Amplitude Modulation) that generates four bits out of one baud has been adopted for flexible modulation, in which the DTX bit allocation as in the QPSK is not adopted. In the HSDPA of the 3GPP standard, a multirate modulation such as 16-QAM or 64-QAM as well as the QPSK, is used. In the case of the multirate modulation, a problem is how to process the DTX bits. In this regard, Mitsubishi has proposed a DTX bit processing method in which DTX bits are bound into one symbol and then mapped into the origin of an IQ plane.
FIG. 1A, FIG. 1B, FIG. 2A, and FIG. 2B illustrate the DTX bit processing method proposed by the Mitsubishi. As shown in FIG. 1A, the empty region of a frame is bound into a 4 bit DTX symbol and then the DTX symbol is mapped to the origin of the IQ plane (see FIG. 1B). In FIG. 2A and FIG. 2B, the empty region of a frame is allocated into each 4 bit symbol to be transmitted by 2 bits such that only 4 symbol points of the IQ plane are used for transmit.
The above DTX bit processing method has a drawback in that this method may not be compatible with the currently developing system since this method requires modification of the current multiplexing scheme. Also, it is very complicated to implement the algorithm in which DTX bits are grouped in one symbol so as to be mapped to the origin or 2 bits are inserted into each data symbol so as to be transmitted using only four symbol points on the IQ plane.